The term "superabsorbent hydrogel-forming materials", as used herein, refers to substantially water-insoluble, absorbent, polymeric compositions that are capable of absorbing large quantities of fluids such as water and body exudates in relation to their weight and forming hydrogels thereby. Such materials are usually also capable of retaining such absorbed fluids under moderate pressures. Superabsorbent hydrogel-forming materials may also be referred to by other names such as "hydrocolloids", or "absorbent gelling materials".
The absorption characteristics of such superabsorbent hydrogel-forming materials, and cost of such materials, makes them especially useful for incorporation into absorbent articles, particularly disposable absorbent articles, such as disposable diapers. Some examples of the use of particulate superabsorbent hydrogel-forming materials in absorbent articles are disclosed in U.S. Pat. No. 3,699,103 issued to Harper et al. on Jun. 13, 1972 and U.S. Pat. No. 3,670,731 issued to Harmon on Jun. 20, 1972.
In the case of some disposable absorbent articles, it is desirable to produce a thinner product. Thinner products may provide user comfort and perception advantages. One way to produce thinner products while maintaining the overall fluid storage capacity of the product is to reduce the fiber weight content in the product's absorbent core while at the same time increasing the amount of particulate superabsorbent hydrogel-forming material in the core. This results in a product having a core with a higher weight concentration of particulate hydrogel-forming material.
However, in historical executions of absorbent articles containing high concentrations of superabsorbent materials (that is, greater than or equal to about 25% by weight), tradeoffs have been inevitable between providing such superabsorbent material concentrations and the fluid uptake and distribution rates of the absorbent articles. Disposable absorbent products containing high concentrations of standard bulk particle size distributions of particulate superabsorbent hydrogel-forming materials have the limitation that their rate of fluid uptake may be much lower than those of conventional cellulosic fiber webs. The term "fluid uptake rate" refers to the rate at which fluids are taken into the core in a direction that is into the plane of the absorbent article (that is, in the "z-direction"). This is particularly true in the case of particulate superabsorbent hydrogel-forming materials that have a relatively large average particle size.
The fluid uptake rate of such absorbent articles can be substantially increased by reducing the average size of the superabsorbent material particles in the product. However, when very small particles (or "fines") swell upon contact with liquids, the particles, when incorporated in a fiber web, tend to be easily forced into the interfiber capillaries of the web. The swollen or partially swollen fines may also form a mass of coagulated gel held together by fluid surface tension forces, thus forming a gel barrier. In either case, resistance to fluid flow through the structure is increased as fluid flow channels are blocked within the fiber web or by the gel mass, resulting in a marked decrease in permeability. These phenomena which interfere primarily with the transportation of fluids in the plane of the absorbent structure (in the "x-y" plane) are commonly referred to as "gel blocking." Further, because these absorbent articles are not able to process fluid (that is, to take in, distribute, and store fluids) rapidly or efficiently, there may be an increased probability of product failure.
There have been many efforts directed at solving the problems associated with incorporating superabsorbent materials into absorbent articles. For example, European Patent Application Publication number 0 339 461 published Nov. 10, 1985, in the name of Kellenberger, describes an effort directed to choosing a size of superabsorbent material that has a specific relationship to the pore size of an absorbent product. U.S. Pat. No. 4,699,823 issued Oct. 13, 1987, in the name of Kellenberger, et al. is directed to distribution of superabsorbent material in a positive concentration gradient through at least a portion of the thickness of an absorbent layer. Other efforts are directed at particle size e.g., U.S. Pat. No. 4,105,033, Chatterjee, et al. Many others mention particle sizes in various contexts, for instance U.S. Pat. Nos. 4,102,340, Mesek, et al.; 4,604,313, McFarland, et al.; and 4,666,975, Yamasaki, et al. However, none of the foregoing appear to have adequately understood and addressed the problems with the transport rate of fluids in both the x-y plane and in the z-direction.
The present invention seeks to resolve the above problems by providing improved absorbent structures and absorbent articles having high concentrations of particulate superabsorbent hydrogel-forming material, with improved fluid uptake and distribution rates.